CI: remove push trigger from Nightly workflow

Nightly should only run on its daily schedule (08:00 UTC) and manual
dispatch, not on every push to master. The push trigger was added in
e2889838ab under the assumption nightly should run per-push; that
duplicates build-all.yml (which already runs on every push) and wastes
~2h of CI plus an unwanted nightly release on every commit.

Surgical edit only -- e2889838ab also added the changelog generator,
dual-core Android build, and PGO profile, which are correct and stay.
build-all.yml is unchanged (still fires on push to master + PR +
workflow_dispatch).

.github/workflows/nightly.yml

@@ -14,11 +14,6 @@
 name: Nightly
 
 on:
-  push:
-    branches: [master]       # build a nightly on every push to master
-    paths-ignore:
-      - '**/*.md'            # skip doc-only pushes
-      - 'docs/**'
   schedule:
     - cron: '0 8 * * *'      # 08:00 UTC daily
   workflow_dispatch: